Method and a device for maintaining an address translation table

ABSTRACT

A device for maintaining an address translation table, placed in series between a user terminal and a third-party entity of a telecommunications network, is disclosed. The device is adapted to verify the existence in the address translation table of an entry specific to the exchange of application signaling messages using said protocol between the terminal and the third-party entity and, if there is no entry specific to the exchange of application signaling messages using said protocol between the terminal and the third-party entity, to create a specific entry in the address translation table associating with a private address and a private port of the terminal in a private network connecting it to said device a public address and a public port of the terminal in the telecommunications network and an indication of the validity of the entry, this validity indication taking into account the first reception time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of the International PatentApplication No. PCT/FR2008/052118 filed Nov. 24, 2008, which claims thebenefit of French Application No. 07 59443 filed Nov. 30, 2007, theentire content of which is incorporated herein by reference.

BACKGROUND

The field of the invention is that of telecommunications networks.

It finds non-limiting preferred applications in Voice over InternetProtocol (VoIP) telecommunications networks.

In Voice over IP networks in particular, users register periodically toinform the network of their presence, thus providing an address at whichthey can be contacted. In such networks in particular, it is usual foruser terminals to access the network via a home gateway. As known initself, this gateway includes a table in which it associates the IPaddress and the port associated with each of the terminals of the homenetwork with an address and a port of that same gateway on the publicnetwork.

This table is known to persons skilled in the art as a network addresstranslation (NAT) table. Here the term NAT is to be understood in abroad sense as encompassing the wider concept of network address andport translation (NAPT).

As is known in the art, gateways regularly delete the content of theaddress translation tables if they are not updated. One known method ofkeeping the address translation tables up to date is to force artificialexchanges of signaling between the user terminal and the network, asdescribed below with reference to FIG. 1.

This figure shows four exchanges of messages L1 to L4 by this mechanism.In this figure:

-   -   the reference 10 designates a terminal;    -   the reference 20 designates a home gateway or a router modem        including a NAT table;    -   the reference 30 designates a user collection unit able to        communicate with the home gateway; and    -   the reference 40 designates a network core unit able to manage        user registration for a given service, for example Voice over        IP, such as a Session Initiation Protocol (SIP) server.

With reference to the exchange of messages L1, the terminal 10 sends thenetwork core entity 40 a request REG (3600) to register for one hour.

These messages pass through the home gateway 20 and the user collectionunit 30, both of which are in series between the terminal 10 and the SIPserver 40.

On reception of this message, the SIP server 40 authorizes maintainingthe registration of the user in its databases for one hour by sending amessage OK (3600).

In the prior art, the value of the subscription provided by the SIPserver is then one hour.

To force the exchange of signaling linked to the registration of theuser of the terminal 10 in order for the information to continue to bestored in the address translation table, the user collection unit 30intentionally reduces the value of the subscription provided by thenetwork by sending the terminal 10 via the home gateway 20 anauthorization message of subscription value that is set at five minutes(300 seconds) in this example.

Consequently, and as shown for the exchange of messages L2,substantially 300 seconds after receiving the message modified by theuser collection unit 30, the terminal 10 requests from the SIP server 40registration for 300 seconds. It is known in the art that the Voice overIP terminals 10 adapt to the authorizations provided by the networkcore.

Of course, it is not necessary for the user collection unit 30 to routethis registration request to the network core 40 because, in the exampledescribed here, the said core has already agreed to maintain thesubscription of the terminal 10 for a subscription period of one hour.

Consequently, the user collection unit 30 merely sends the terminal 10 anew subscription authorization for a further 300 seconds.

The mechanism described for the exchange of messages L2 is repeatedidentically for the exchange of messages L3, as shown by thedouble-headed dashed-line arrow.

As is known in the art, the user collection unit 30 monitors in parallelwith the above process whether the subscription provided by the SIPserver 40 (which is one hour in this example) is approaching its expiry.

Assume that this is so when the user collection unit 30 receives therequest for registration for five minutes during the exchange ofmessages L4.

The user collection unit 30 then replaces the value of the subscriptionminus 300 seconds in this registration message with the originalsubscription value of 3600 seconds, in order to solicit a newsubscription of the terminal 10 with the SIP server 40 of one hour.

As described above with reference to the exchange of messages L1, theSIP server 40 agrees to a subscription of one hour and the usercollection unit 30 substitutes in this message for the value of thesubscription provided by the network core a reduced value set at 300seconds, in order artificially to force exchange of signaling by theterminal 10 to maintain the address translation table in the homegateway 20 up to date.

A first drawback of the above mechanism is that it artificiallygenerates superfluous application signaling message traffic in theaccess network, which obliges the operator to double the capacity of theVoice over IP service user collection units.

A second drawback of this mechanism is that it requires the introductioninto the collection units of functions specific to generating thisartificial traffic.

SUMMARY

There is therefore a requirement to propose a simpler and more effectivesolution for maintaining an address translation table of a home gatewayor a router modem, in particular without generating superfluous messagesin the access network.

This is achieved by a device for maintaining an address translationtable, placed in series between a user terminal and a third-party entityof a telecommunications network, the device being characterized in thatit is adapted:

-   -   to intercept a registration request message sent by the terminal        to said third-party entity using an application signaling        protocol, said message being received by said device at a first        reception time at a reception port dedicated to the exchange of        messages using said protocol;    -   to verify the existence in the address translation table of an        entry specific to the exchange of application signaling messages        using said protocol between the terminal and the third-party        entity; and    -   if there is no entry specific to the exchange of application        signaling messages using said protocol between the terminal and        the third-party entity, to create a specific entry in the        address translation table associating with a private address and        a private port of the terminal in a private network connecting        it to said device a public address and a public port of the        terminal in the telecommunications network and an indication of        the validity of the entry, this validity indication taking into        account the first reception time and a first validity period.

With the assistance of one of the input ports dedicated to applicationsignaling messages, the device of the invention for maintaining anaddress translation table recognizes that the message sent by theterminal to the third-party entity is a request message conforming to aparticular application signaling protocol. This protocol corresponds toa specific type of entry in the address translation table with which itassociates a validity indication specific to that entry.

Thus the invention is based on an entirely novel and inventive approachto maintaining an address translation table that relies on the abilityof the device to recognize the application signaling protocol used inexchanges of messages between the terminal and the third-party entityand to create in the address translation table a specific entryconcerning the exchanges of messages conforming to that protocolincluding a validity indication suited to that type of entry.

According to a first aspect of the invention, the first validity periodhas a predetermined value that is specific to said application signalingprotocol. For example, this first validity period is configured to theusual duration of a registration subscription delivered by thethird-party entity. An advantage of this is that, usually, the firstvalidity indication corresponds to a registration period agreed by thethird-party entity, which limits recourse to the superfluous messages ofthe prior art.

According to a second aspect of the invention, the registration requestsent by the user terminal includes a required registration validityperiod and the first validity period is chosen to be equal to saidrequired registration validity period.

An advantage of this is that the validity indication is specificallyadapted to the user terminal registration request. Accordingly, if therequest for the required registration period is accepted, the validityperiod of the entry corresponds to the duration of the registrationsubscription. Under such circumstances, it is therefore no longernecessary to have recourse to the prior-art mechanism for sending thesuperfluous messages.

According to a third aspect of the invention, the device for maintainingan address translation table is adapted:

-   -   to await an authorization message sent by the third-party entity        to the public port of the user terminal in response to said        request;    -   on reception at a second reception time of an authorization        message from the third-party entity to the terminal, to extract        a registration validity period, if there is one; and    -   if there is a validity period and it is not zero, to update the        validity indication of the entry on the basis of the second        reception time and the extracted registration validity period.

According to this third aspect of the invention, the validity indicationof the entry specific to exchange of application signaling between theterminal and the third-party entity in accordance with a particularprotocol is produced in two stages:

-   -   firstly, at the time of creating the entry and on the basis of a        first validity period; this first validity period can have any        value, for example a predetermined value of a few minutes or the        value of the validity period of the registration requested by        the user terminal, as referred to above; and    -   secondly, when it intercepts the authorization message sent by        the third-party entity, which is preferably a network input        entity such as, for example, a proxy call session control        function (P-CSCF) type application server in an IP Multimedia        Subsystem (IMS) architecture, the device of the invention        extracts therefrom a validity period of the registration; it        uses this registration validity period to update the validity        indication of the entry in the table.

As a result, the validity indication of the entry in the addresstranslation table is updated on the basis of the registration orsubscription period actually supplied by the application server. Anadvantage of this is that the device of this third aspect of theinvention guarantees that maintaining the entry in the addresstranslation table is in phase with a successful registration.

Another advantage is that it means that a short first validity periodcan be chosen, for example a few minutes, at the same time as avoidingthe user terminal sending superfluous signaling messages for maintainingthe address translation table up to the end of the subscription. Onebenefit of choosing a first validity period of a few minutes rather thana registration subscription period (at least one hour) is to ensurefaster deletion of the entry from the address translation table in theevent of the third-party entity rejecting the request from the userterminal.

Consequently, and in contrast to the prior art, the collection entity nolonger has any role to play in maintaining the address translationtable. In particular, it no longer has to detect a network addresstranslation situation (NAT situation) or to substitute for theregistration validity period value a lower value corresponding to thevalidity period of a few minutes usually assigned to a standard entry ofan address translation table.

According to another aspect of the invention, the validity indication ofthe entry is updated by adding the validity period of the registrationto the second reception time.

An advantage of this is that the entry specific to exchange of messagesin accordance with the application signaling protocol between theterminal and the telecommunications network indicates the expiry time ofthe entry by means of a single value.

Alternatively, the validity indication can include a validity period ofthe entry equal to the validity period of the authorization and a timefrom which the validity period applies, for example the second receptiontime.

According to another aspect of the invention, if an entry specific tothe exchange of application signaling messages using this protocolbetween said terminal and the telecommunications network already existsin the address translation table, said device relays the applicationsignaling message directly to the third-party entity and awaits anauthorization message.

According to another aspect of the invention, on reception of anauthorization message including a zero registration validity period orno registration validity period, the device for maintaining an addresstranslation table can delete the entry from the address translationtable.

One advantage of this is that the entries of the translation tablespecific to exchanges of application signaling messages in accordancewith the invention are freed as quickly as possible, which enablesoptimization of the resources of the address translation table.

In a correlated way, the invention provides a method of maintaining anaddress and ports translation table adapted to be implemented by adevice placed in series between a user terminal of a private network anda third-party entity of a telecommunications network, the method beingcharacterized in that it includes the following steps:

-   -   intercepting a registration request message sent by the terminal        to said third-party entity using an application signaling        protocol, said message being received by said device at a first        reception time at a reception port dedicated to exchanges of        messages using said protocol;    -   verifying the existence in the address translation table of an        entry specific to the exchange of application signaling messages        using said protocol between the terminal and the third-party        entity; and    -   if there is no entry specific to the exchange of application        signaling messages using said protocol between the terminal and        the third-party entity, creating a specific entry in the address        translation table associating with a private address and a        private port of the terminal in a private network connecting it        to said device a public address and a public port of the        terminal in the telecommunications network and an indication of        the validity of the entry, this validity indication taking into        account the first reception time.

According to one aspect of the invention, the method further includesthe following steps:

-   -   awaiting an authorization message sent by the third-party entity        to the user terminal in response to said request;    -   on reception at a second reception time of an authorization        message from the third-party entity to the terminal, extracting        a registration validity period, if there is one; and    -   if there is a validity period and it is not zero, updating the        validity indication of the entry on the basis of the second        reception time and the registration validity period.

In one particular implementation, the steps of the method of maintainingan address translation table are determined by computer programinstructions.

Consequently, the invention also provides a computer program on aninformation medium, that program being adapted to be executed in adevice for maintaining an address translation table or more generally ina computer and including instructions adapted to execute the steps of amethod as described above for maintaining an address translation table.

This program may use any programming language and may take the form ofsource code, object code or a code intermediate between source code andobject code, such as a partially-compiled form, or any other desirableform.

The invention also provides a computer-readable information mediumincluding instructions of a computer program as referred to above.

The information medium may be any entity or device capable of storingthe program. For example, the medium may comprise storage means, such asa ROM, for example a CD ROM or a micro-electronic circuit ROM, ormagnetic storage means, for example a floppy disk or a hard disk.

Moreover, the information medium may be a transmissible medium such asan electrical or optical signal, suitable for being routed via anelectrical or optical cable, by radio, or by other means. The program ofthe invention may in particular be downloaded over an Internet-typenetwork.

Alternatively, the information medium may be an integrated circuitincorporating the program, the circuit being adapted to execute themethod in question or to be used in its execution.

The device and the method of the invention may in particular be executedby a home gateway or a router modem.

Consequently, the invention further provides a home gateway or routermodem for connecting a user terminal of a private network to athird-party entity of a telecommunications network, the gateway or modembeing characterized in that it includes:

-   -   an address translation table able to store an entry specific to        the exchange of application signaling messages between the user        terminal and the third-party entity of the telecommunications        network using an application signaling protocol, said entry        associating a private address and a private port of the terminal        in the private network with a public address and a public port        of the terminal in the telecommunications network and an        indication of the validity of said entry; and    -   a device for maintaining the address translation table as        described above.

The invention finally provides an address translation table adapted toassociate in an entry a private address and a private port of a terminalin a private network with a public address and a public port of thatterminal in a telecommunications network, the table being characterizedin that it includes an entry specific to the exchange of applicationsignaling between the user terminal and a third-party entity of thetelecommunications network using an application signaling protocol, saidentry further including an indication of the validity of said entry.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention become more clearlyapparent on reading the following description by way of illustrative andnon-limiting example only of one particular embodiment of the inventionand from the appended drawings, in which:

FIG. 1 shows diagrammatically a prior art mechanism for maintaining anaddress translation table;

FIG. 2 shows diagrammatically and in its environment a device of oneembodiment of the invention for maintaining an address translationtable;

FIG. 3 shows in flowchart form steps of a method of the invention formaintaining an address translation table; and

FIG. 4 shows messages exchanged in the context of the invention asdescribed with reference to FIG. 3.

DETAILED DESCRIPTION

The general principle of the invention is to use a mechanism formaintaining an address translation table that is specific to the entriesin the table that relate to using an application signaling protocol toexchange application signaling messages between a user terminal of aprivate network and a third-party entity of a telecommunicationsnetwork.

There is considered below the example of a user terminal seeking toaccess a Voice over IP (VoIP) service via an IMS network. The userterminal therefore sends a service registration request to an accessentity of the IMS network using an application signaling protocol, forexample the Session Initiation Protocol (SIP).

Note that the invention is not limited to this example, however, andthat the mechanism of the invention for maintaining an addresstranslation table can be triggered following a user terminal sending aregistration request using any other application signaling protocol, forexample the H.323 protocol, the H.248 protocol or the Media GatewayControl Protocol (MGCP).

Below, the expression registration request must be interpreted in abroad sense, i.e. as not limited to an initial registration request butalso encompassing a registration renewal request or a deregistrationrequest. Note also that some application signaling protocols specifyother types of registration request. This is the situation with the SIP,for example, in which a user terminal sends an active subscriptionslisting registration request enabling it to obtain in response thevalidity period or remaining subscription period for each contactaddress.

FIG. 2 shows a user terminal 10 connected to a private network (homenetwork) 1. The private network of the user also includes another userterminal 11 and a computer 12. The customer installation is connected toa collection network 2 of the operator via a home gateway or a routermodem 200. For simplicity, the expression home gateway is used below torefer to both these devices. It should nevertheless be borne in mindthat the method of the invention can be implemented by any equipmenthaving the router modem function.

A collection unit 30 relays a registration request sent by the userterminal 10 using the SIP to an application server 40 of the corenetwork 3 implementing an IMS architecture. The expression applicationserver refers to any entity of the telecommunications network able toexchange application signaling messages directly with the user terminal10, for example a P-CSCF type IMS network SIP access entity. Theregistration request is therefore relayed to another entity of the IMSnetwork hosting a REGISTRAR function, for example a Serving-Call SessionControl Function (S-CSCF) entity.

The home gateway 200 contains an address translation table 26.

The standard function of an address translation table is to translatethe combination (private address @LAN10, private port P10) of the userterminal 10 into the combination (public address @PU20, public portP20).

According to the invention, the entry 27 is specific to a succession ofexchanges of application signaling messages using a given signalingprotocol between the terminal 10 and a third-party entity such as theapplication server 40. It is created as soon as the user terminalrequests to communicate with the application server 40 by sending aregistration request to the IMS network using the signaling protocol.According to the invention, the entry 27 includes a validity indicationED in addition to the private and public (address, port) combinations.The entry 27 of the invention is therefore a quintuplet rather than aquadruplet as in the prior art.

The home gateway 200 further includes a device of the invention formaintaining the address translation table 20.

The home gateway 200 finally includes at least one receiver port 201dedicated to receiving application signaling messages using the givenprotocol coming from one of the terminals 10, 11, 12 of the privatenetwork of the user.

In the embodiment of the invention described here, the device 20 of theinvention includes hardware components conventionally found in astandard computer, namely a processor 22, a random-access memory (RAM)23, a read-only memory (ROM) 24, telecommunications means 21 forcommunicating with the private network 1, and telecommunications means25 for communicating with the public network 2.

The device 20 of the invention is connected to the address translationtable 26, in which it can create or delete entries.

The read-only memory 24 constitutes a storage medium that stores acomputer program of the invention including instructions for executingthe steps of a method of the invention for maintaining an addresstranslation table as described below with reference to FIGS. 3 and 4.

The method of maintaining an address translation table used by thedevice 20 includes a step El of intercepting a request to register forthe Voice over IP (VoIP) service sent by the terminal 10 to theapplication server 40 and received at a home gateway receiver port 201dedicated to the SIP application signaling protocol at a first receptiontime DR.

In a step E2, it verifies the existence in the address translation table26 of an entry 27 specific to the exchange of signaling messages betweensaid terminal 10 and the network 3 using the SIP.

If there is no entry in the address translation table 26 specific toapplication signaling message exchanges of this type, the method of theinvention, in a step E3, creates a specific entry 27 in the addresstranslation table 26 associating with a private address and a privateport (@LAN10, P10) of the terminal 10 in its private network 1 a publicaddress and a public port (@PUB20, P20) of the terminal in thecollection network 2 and a validity indication, for example a firstexpiry time ED of the entry starting from the first reception time DR.It calculates the first expiry time ED by adding a first validityperiod, for example. Alternatively, the validity indication includes astart time and a first validity period.

According to a first aspect of the invention, this first validity periodcan have a predetermined value; according to a second aspect of theinvention, it can be a requested registration validity period if therequest from the user terminal specifies one.

According to a third aspect of the invention, the first validity periodis updated in two stages. Firstly, the device for maintaining an addresstranslation table assigns it a value of a few minutes, for example 300seconds, at the reception time DR.

Secondly, in a step E4, it relays the application signaling message tothe collection unit 30 and then waits for a response message from theapplication server 40 to the terminal 10.

If an entry specific to the exchange of VoIP type application signalingmessage exists in the address translation table 26 for the user terminal10, the device of the invention goes directly to the step E4.

At a second reception time, in a step E5, the device of the inventionintercepts a response message coming from the SIP server 40 and receivedat the port P20 of the home gateway 200.

If it is an authorization message, it indicates a registration validityperiod granted to the terminal by the application server 40, for examplea validity period for an SIP registration. The SIP standard (RFC3261)specifies a default validity period of one hour, for example. For itspart, the 3GPP standard specifies for the IMS system a default value ofone week. Note that the invention is not limited to these examples,however, and that it applies to any other value chosen by thetelecommunications network operator.

In a step E5, the device of the invention extracts the registrationvalidity period from the received authorization message.

If it is a rejection message, the first expiry time ED is not updated.The entry 27 is deleted from the address translation table 26 when theexpiry time ED is reached.

If it is a response message to an explicit deregistration request, itmay indicate either a zero validity period or no period.

If it is a response message to an active subscriptions listingregistration request, it may indicate one or more non-zero validityperiods or no period.

In a step E6, the device of the invention tests the value of theextracted validity period. If it is not zero, it calculates in a stepE27 a new expiry time E7 of the entry from the address translation tableby adding the validity period to the second reception time. It thenupdates the quintuplet of the specific entry 27, replacing the oldexpiry time with the calculated expiry time.

If the validity period is zero or there is no period in the receivedresponse message, the device of the invention deletes the entry 27 fromthe address translation table 26 during a step E8.

Message exchanges L1 to L3 in one implementation of the invention aredescribed below with reference to FIG. 4.

Assume, for example, that in a first application signaling messageexchange L1 the user terminal 10 sends the application server 40 an SIPregistration request REG (3600) to register for a service. This requestis passed on to the application server 40 in succession by the homegateway 200 including the device of the invention and the usercollection unit 30.

This request optionally specifies a required registration validityperiod, for example 3600 seconds.

This request triggers the mechanism of the invention for maintaining theaddress translation table.

The device 20 of the invention in the home gateway 200 intercepts thereceived message at the dedicated port 201 at the reception time T1. Itconsults the address translation table 26 of the home gateway to verifywhether there exists for the terminal 10 a message specific toexchanging application signaling messages using the VoIP protocol. Ifso, it relays the message REG(3600) to the collection unit 30. If not,it creates an entry 27 in the address translation table 26 into which itwrites the quintuplet (@LAN10, P10, @PUB20, P20, ED) using the mechanismof the invention described above. The validity indication is calculatedfrom the time the device 20 receives the request REG and a firstvalidity period, which can have a predetermined value or the value ofthe requested registration validity period.

According to one aspect of the invention, the device 20 then begins towait for a response message from the third-party entity.

When at reception time T2 it receives a response message OK (3600) atthe public port P20 created by the home gateway 200 for exchanges of theuser terminal 10 with the public network, it intercepts it and extractsfrom it a validity period DV, if there is one.

If this period is not zero, it updates the expiry time in the entry 27of the address translation table by T2+DV and then relays the responseOK (3600) to the terminal 10.

During a second message exchange L2 the terminal 10 sends a renewalrequest RRE (3600) to the application server 40 before the expiry timeT2+3600 is reached.

On reception of this request at the dedicated port 201 of the homegateway 200, the device 20 of the invention verifies whether an entryfor the user terminal 10 exists in the address translation table 26. Ifso, a plurality of options are open:

According to one aspect of the invention, the device 20 updates thevalidity indication of the entry using a predetermined validity periodvalue, preferably a usual registration subscription value.

According to another aspect of the invention, it extracts a requestedregistration validity period from the renewal request, if there is one,and updates the validity indication of the entry on the basis of thetime the renewal request is received and the requested registrationvalidity period.

According to a further aspect of the invention, it relays the requestRRE (3600) directly to the collection unit 30 and awaits a responsemessage. When the response message OK (3600) arrives at reception timeT3, it updates the entry 27 of the address translation table byreplacing the old validity indication with an updated value obtainedfrom T3 and a validity period DV indicated in the response message, andthen relays the response message to the terminal 10.

During a third message exchange L3, the terminal 10 sends the SIP server40 an explicit deregistration request RDE before the expiry time T3+DVis reached.

According to one aspect of the invention, because an entry exists in theaddress translation table 26, the device of the invention relays thisrequest directly. On reception of the authorization response from theSIP server 40 at reception time T4, it extracts from the message a zerovalidity period or notes the absence of a validity period and deletesthe entry 27 from the address translation table.

The invention claimed is:
 1. A device for maintaining an addresstranslation table, placed in series between a user terminal and athird-party entity of a telecommunications network, the devicecomprising a hardware processor programmed: to intercept a registrationrequest message sent by the user terminal to the third-party entityusing an application signaling protocol, the registration requestmessage being received by the device at a first reception time at areception port dedicated to the exchange of messages using theapplication signaling protocol; to verify the existence in the addresstranslation table of an entry specific to the exchange of applicationsignaling messages using the application signaling protocol between theuser terminal and the third-party entity; and if there is no entryspecific to the exchange of application signaling messages using theapplication signaling protocol between the user terminal and thethird-party entity: to create a specific entry in the addresstranslation table associating the following with a private address and aprivate port of the user terminal in a private network connecting theterminal to the device: a) a public address and a public port of theuser terminal in the telecommunications network: and b) an indication ofthe validity of the specific entry, wherein: the validity indication ofthe specific entry takes into account the first reception time and afirst validity period, and the first validity period has a predeterminedvalue that is specific to the application signaling protocol.
 2. Thedevice for maintaining the address translation table according to claim1, wherein the registration request message sent by the user terminalincludes a requested registration validity period, and the firstvalidity period is chosen to be equal to the requested registrationvalidity period.
 3. The device for maintaining the address translationtable according to claim 1, wherein it is further configured: to awaitan authorization message sent by the third-party entity to the publicport of the user terminal in response to the registration requestmessage; on reception at a second reception time of the authorizationmessage from the third-party entity to the user terminal, to extract aregistration validity period, if there is one; and if the registrationvalidity period exists and it is not zero, to update the validityindication of the specific entry based on the second reception time andthe extracted registration validity period.
 4. The device formaintaining the address translation table according to claim 3, whereinthe updated validity indication of the specific entry is calculated byadding the extracted registration validity period to the secondreception time.
 5. The device for maintaining the address translationtable according to claim 3, wherein, if the entry specific to theexchange of application signaling messages relating to the user terminalalready exists in the address translation table, the device relays theregistration request message directly to the third-party entity andawaits the authorization message in response to the registration requestmessage.
 6. The device for maintaining the address translation tableaccording to claim 3, wherein, on reception of the authorization messageincluding a zero registration validity period or no registrationvalidity period, the device can delete the specific entry from theaddress translation table.
 7. A home gateway or router modem forconnecting a user terminal of a private network to a third-party entityof a telecommunications network, the home gateway or router modemcomprising: an address translation table configured to store an entryspecific to the exchange of application signaling messages between theuser terminal and the third-party entity of the telecommunicationsnetwork using an application signaling protocol, the entry specific tothe exchange of application signaling messages associating a privateaddress and a private port of the user terminal in the private networkwith a public address and a public port of the user terminal in thetelecommunications network, and an indication of the validity of theentry specific to the exchange of application signaling messages; and acode for maintaining the address translation table configured: tointercept a registration request message sent by the user terminal tothe third-party entity using the application signaling protocol, theregistration request message being received at a first reception time ata reception port dedicated to the exchange of messages using theapplication signaling protocol; to verify the existence in the addresstranslation table of the entry specific to the exchange of applicationsignaling messages using the application signaling protocol between theuser terminal and the third-party entity; and if there is no entryspecific to the exchange of application signaling messages using theapplication signaling protocol between the user terminal and thethird-party entity; to create a specific entry in the addresstranslation table associating the following with the private address andthe private port of the user terminal in the private network connectingthe terminal to a device a) the public address and the public port ofthe user terminal in the telecommunications network; and b) anindication of the validity of the specific entry, wherein: the validityindication of the specific entry takes into account the first receptiontime and a first validity period, and the first validity period has apredetermined value that is specific to the application signalingprotocol.
 8. A non-transitory computer readable medium storing anaddress translation table configured to store an entry relating to auser terminal, the entry associating a private address and a privateport of the user terminal in a private network with a public address anda public port of the user terminal in a telecommunications network, theaddress translation table comprising: an entry specific to an exchangeof application signaling between the user terminal and a third-partyentity of the telecommunications network using an application signalingprotocol, the entry specific to the exchange of application signalingfurther comprising an indication of the validity of the entry specificto the exchange of application signaling, the validity indication of thespecific entry taking into account a first reception time and a firstvalidity period, wherein the first validity period has a predeterminedvalue that is specific to the application signaling protocol.
 9. Amethod of maintaining an address translation table configured to beimplemented by a device placed in series between a user terminal of aprivate network and a third-party entity of a telecommunicationsnetwork, the method comprising the steps of: intercepting a registrationrequest message sent by the user terminal to the third-party entityusing an application signaling protocol, the registration requestmessage being received by the device at a first reception time at areception port dedicated to exchange of application signaling messagesusing the application signaling protocol; verifying the existence in theaddress translation table of an entry specific to the exchange ofapplication signaling messages using the application signaling protocolbetween the user terminal and the third-party entity; and if there is noentry specific to the exchange of application signaling messages usingthe application signaling protocol between the user terminal and thethird-party entity: creating a specific entry in the address translationtable associating the following with a private address and a privateport of the user terminal in the private network connecting the terminalto the device: a) a public address and a public port of the userterminal in the telecommunications network; and b) an indication of thevalidity of the specific entry, wherein: the validity indication of thespecific entry takes into account the first reception time and a firstvalidity period, and the first validity period has a predetermined valuethat is specific to the application signaling protocol.
 10. The methodof maintaining an address translation table according to claim 9, themethod further comprising the steps of: awaiting an authorizationmessage sent by the third-party entity to the public port of the userterminal in response to the registration request message; on receptionat a second reception time of the authorization message from thethird-party entity to the public port of the user terminal, extracting aregistration validity period, if there is one; and if the registrationvalidity period exists and it is not zero, updating the validityindication of the specific entry based on the second reception time andthe registration validity period.
 11. A non-transitory computer readablemedium storing a computer program comprising instructions for executingthe method of maintaining the address translation table according toclaim 9, when the computer program is executed by a computer.